mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Maize
There are several different types of transgenic corn, but the two most common are herbicide resistant and insecticide-producing varieties, which are reviewed below. Herbicide Resistant This transgenic line has been modified to have resistance to herbicides. Particularly, lines have been developed to be resistant to N-(phosphomethyl)glycine, also known as glyphosate. This herbicide is a common agent used to kill weeds, originally patented as “roundup” by Monsanto. Glyphosate works by inhibiting a particular enzyme important for the production of phenylalanine, tyrosine, and tryptophan. Therefore, the development of actively growing plants is effected with the use of glyphosate. Herbicide resistance is accomplished by glyphosate tolerance. This was achieved by removing the ability for glyphosate to inhibit 5-enol pyruvylshikimate-3-phosphate synthase (2mEPSPS). This is done through creation of a double mutant 2mEPSPS protein, which prevents glyphosate from binding (Herouet-Guicheney et al, 2009). Specifically, a coding sequence of 2mEPSPS was isolated and site-directed mutagenesis was preformed. Position 102 was mutated from a threonine to an isoleucine, and position 106 was mutated from a proline to a serine. Insecticide-Producing Bacillus thuringiensis ''expresses two toxins that have been utilized in genetically modified organisms. Crystal (Cry) and Cytolitic (Cyt) proteins are produced by the bacteria which are toxins to many types of insects including Diptera, Coleoptera, Lepidoptera, as well as the nematode (Bravo et al., 2007). Cry proteins are a class of pore-forming toxins that interact with the cell surface of the host. They are water-soluble proteins secreted by Bacillus thuringiensis. This process is initiated with binding to host cell receptors, and is activated by host cell proteases (Bravo et al., 2007). Specifically, Cry proteins target the gut of the insect by lysing epithelial cells in this organ. Cry1A proteins are a subgroup that is commonly used in genetically modified plants.Examples if particular receptors that Cry1A proteins bind to are highlighted in Figure 1. Cryt proteins act in a different fashion by direct interaction with lipids of the host cell membrane. MON810 YieldGard Monsanto maize were genetically modified to express insecticidal proteins through the introduction of the Cry1(A)b gene . The transgene was originally introduced into the plant as a cassette with a cauliflower virus 35S promoter, a hsp70 intron sequence, and the Cry1(A)b gene (Rosati et al., 2008). The original transgenic production of this line showed that the Cry1(A)b gene was being truncated. Further studies showed that this was a recombination event, but didn’t actually affect the overall activity of the Cry1A protein (Rosati et al., 2007). One example of an insect that this transgenic corn line targets is the European corn borer (Ostrinia nubilalis), shown in Figure 2. Original Intentions ''For the herbicide resistant transgenic line: the use of herbicides would not kill off this transgenic line, but would impinge on weed growth. Glyphosate is not toxic and is environmentally friendly (Duke and Powles, 2006). For the insecticide-producing transgenic line: by genetically modifying the corn to produce insecticidal proteins, less emphasis could be agriculturally put on spraying insecticides. Concerns One concern that has been associated with the development of the herbicide resistant transgenic corn, as well as spraying herbicides in general, is the development of insect resistance to these broad-spectrum agents. This is shown by the fact that the use of glyphosate has had a six-fold increase between the years of 1992 and 2002 (Cerdiera et al., 2006). Glyphosate is now the most highly used herbicide in the United States, largely due to the development of genetically modified plants with a resistance to this agent. The increasing resistance of insects is quite concerning for the future of crop management. With the use of the Gly protein from Bacillus thuringiensis in the insecticide-producing transgenic corn line, one potential concern is horizontal gene transfer. Studies are continuing to be conducted as preventative measures. Over the years, there has been a controversy over the use of genetically-modified organisms. Much of the general public is not well educated as to what genetically modified organisms really entail. Instead viewing the products in a negative light, believing that anything modified in the organism can be transferred to humans by eating the product. References Bravo A., Gill S.S., and Soberon M. Mode of action of Bacillus thuringiensis Cry and Cyt toxins and their potential for insect control. Toxicon 2007 (49) 423-435. Cerdeira AL. and Duke SO. The current status and environmental impacts of glyphosate-resistant crops: a review. Jour of environmental quality 2006 (35) 1633-1658. Duke SO. and Powles SB. Glyphosate: a once in a century herbicide. Pest management science 2008 (64) 319-325. Rosati A., Bogani P., and Santarlasci A. Characterisation of 3’ transgene insertion site and derived mRNAs in MON810 YieldGard maize. Plant Mol Biol 2008 (67) 271-281. Herouet-Guicheney C., Rouquie D., Freyssinet M., Currier T., Martone A., Xhou J., Bates E., Ferullo J., Hendrickx K., and Rouan D. Safety evaluation of the double mutant 5-enol pyruvylshikimate-3-phosphate synthase (2mEPSPS) from maize that confers tolerance to glyphosate herbicide in transgenic plants. Pharmacology 2009 (54) 143-153. Genetically modified maize wiki European corn borer wiki